I haven't had any luck with GTKRadiant 1.6 on *any* of my 64 bit win 7 machines so I stick with 1.4; something about my wacom tablet and my Expert Mouse trackball (both are serial devices) seem to play hell with scrolling.
I'd check on irc.quakenet.org #radiant for windows 8 problems, I don't use it at home for anything. Yet.